Traditional impedance matching devices are usually based on either the ".pi." network or the "T" network, both of which are well known electronic circuits, and which are illustrated in FIGS. 1(a) and 1(b), respectively.
In the .pi. network of FIG. 1(a) (so called because the network diagram resembles the Greek letter .pi.), impedance matching is achieved by a single series impedance Z2 and two parallel impedances Z1 and Z3 to ground, one located at the input and one located at the output of Z2. Typically, series impedance Z2 is modelled as an inductor L, while parallel impedances Z1 and Z2 are modelled as capacitors C.sub.1 and C.sub.2, respectively. The impedances may be fixed, but it is usually preferred that they be variable in order to give the circuit a range in frequencies over which the impedances may be matched. Thus, the .pi. network of FIG. 1(a) is illustrated as comprising a series impedance in the form of a variable inductor and parallel impedances in the form of variable capacitors.
In the T network (so called because it resembles the letter T), impedance matching is achieved by using two series impedances Z1 and Z3 and a single parallel impedance Z2 to ground located at the node between Z1 and Z3. In the T network, the series impedances are also modelled as variable inductors L, and L.sub.2 and the parallel impedance as a variable capacitor, as illustrated in FIG. 1(b).
The .pi. and T networks form the building blocks for most conventional impedance matching circuits. They are well understood, can be modelled using existing computer-aided design methods, and can be used to form other, more complex impedance matching circuits. Although used in many conventional antenna coupler configurations, the .pi. and T matching sections present certain drawbacks for applications, such as an antenna coupler.
If large magnitude impedance transformations are required, it is possible to develop extremely high RF potentials on one or more of the matching networks. For this reason it is often necessary to, in the case of a .pi. section device, choose a capacitor or switch, for example, capable of withstanding voltages in excess of 10,000 volts. In some cases, this voltage can ionize the air and cause a shorting path. For this reason, vacuum capacitors and relays such as those manufactured by the Jennings Corporation are often used. Solid state circuits matching circuits based on a .pi. or T configuration can expose the solid state switches to extreme current and voltage conditions especially when attempting to match a "short antenna" at the low end of the HF frequency range (HF is typically 2 to 30 MHz). For example, when using a .pi. section for matching to a 15 foot monopole antenna, a current as high as 10 amperes may flow through the inductor Z.sub.2, when attempting to transmit 1 to 2 kw power due to the typical impedance of this antenna varying from 0.5-j 900 ohms at 2 MHz to 400+j 500 ohms at 15 MHz. The inductor of the matching network may be shorted-out during a portion of this frequency range in order to obtain the needed impedance matching which, in turn, may cause its related solid state switch to experience a current as high as 20 amperes for this short duration. Furthermore, the voltage across the inductor at the 2 megahertz frequency may be in excess of 10 KV, and unless each turn forming the inductor is switched separately, the required voltage rating for the solid state switches, in their off-state, may be substantially over 1 KV. These high current and voltage conditions caused by the .pi. as well as the T matching sections impose high stress on the solid state devices. Also, because it is desired to mount the switches in close proximity to the inductor for high frequency applications in order to overcome the disadvantages of switch lead inductance, a serious thermal problem may arise because of the heat transferred from the inductor to the solid-state switch. These high voltage, current, and thermal conditions may contribute to the premature failure of the solid state devices.
There is a need for an impedance matching device that overcomes the drawbacks of the conventional impedance matching .pi. or T circuits. In particular, there is a need for an impedance matching device that reduces or even eliminates the high voltage stresses that are placed on the switches used at high power and high frequency applications to prevent the use of semiconductor switches such as diodes or the switch described in U.S. Pat. No. 4,808,859. Further, the impedance matching device should perform over a wide range of frequencies so that the related antenna or other device can be properly matched to the desired impedance.
In addition to overcoming the drawbacks of the .pi. and T matching sections, there is a need to minimize the number of solid state switches associated with impedance matching devices. The numbers of switch devices are directly related to the number of switchable segments of the matching sections, wherein one switching device operatively connects or disconnects a corresponding switchable segment. It is desired that means be provided to reduce the number of switching devices needed for impedance matching devices.
Accordingly, it is an object of the present invention to provide impedance matching devices that reduce the number of switching devices, especially for high power and high frequency applications.
It is another object of the present invention to provide impedance matching devices that overcome the drawbacks of conventional impedance matching .pi. or T circuits related to the high current, voltage and thermal stress conditions of the switching devices.